High temperatures and humidity are likely to make Sunday's Virgin London Marathon one of the hottest ever.
Forecasters are predicting the warmest temperatures of the year this weekend with the mercury set to hit 21C in the South East and London over the weekend.
That would make Sunday's race the third hottest after the 1996 and 2007 marathons.
But light showers could provide some relief from the warm temperatures.
The sunshine will return to the South East on Monday with forecasters predicting a warm start to next week.